The general idea is that, if the estimator is smooth enough, for example if it is Fréchet or Hadamard differentiable, see [1]}, then one can write the following expansion
\(T(P)=T(Q)+\int _{\mathbb {R}^d} \mathrm {IF}(x,T,Q)d(P-Q)(x)+R(P,Q),\) 
